The Hypertext Transfer Protocol (HTTP) is an application protocol for distributed information systems and is the foundation of data communication for the World Wide Web. Caching HTTP objects (html files, video, audio, images, etc.) is a well known technique to improve performance of applications requesting the objects. The HTTP/1.1 protocol (and later) includes a number of elements intended to make caching work efficiently. The goal of caching is to reduce, wherever possible, the need to send requests, and when a request must be sent, to reduce the need to send full responses in reply. Reducing the need to send requests lowers the number of network round-trips required, and is typically accomplished through “expiration” mechanisms allowing the requesting application or computer to assume that a cached copy of a resource is current if within a specified validity period. Reducing the need to send full responses lowers the strain on network bandwidth, and is typically accomplished through “validation” mechanisms. When a cache has a stale entry that it would like to use as a response to an application's or client's request, it first has to check with the origin server (or possibly an intermediate cache with a fresh response) to see if the cached (stale) entry is still usable. This is known as “validating” the cache entry. Known methods of validation include the use of “last-modified dates” and “entity tag” (ETag) values included in HTTP headers.
Intermediate “caching” servers that can store resources locally so that subsequent requests for the resource can be returned directly from the intermediate server often have the capability to pre-fetch a resource in anticipation of the resource being requested by a client computer.